The present invention relates to vacuum drying kilns, and particularly to drying kilns for timber, of the kind comprising at least one drying chamber, a vacuum pump connected to the said chamber and operable to produce in it a depression for the purpose of evaporating moisture contained in the wood, and a heating element operable to heat the interior of the chamber, and hence to raise the temperature of timber placed therein.
It is known that the heat required for drying timber, whatever the drying system used, consists of a first quantity of heat for evaporating the free water of the wood (known as the vaporisation heat) and a second quantity of heat for freeing the water which impregnates the cellular membranes (known as hygroscopic heat).
The relative quantities of evaporation heat and hygroscopic heat are very important in drying timber. More exactly, the quantity of vaporisation heat Q, (generally, but not always, expressed with reference to a kilogram of water evaporated) is related to the drying temperature T, by the formula: EQU Q=606.5-0.695 T Cal/Kg of water evaporated.
For example, if the operating temperature is 60.degree. C., then the quantity of vaporisation heat corresponds to 555 calories per evaporated liter of water for any kind of unseasoned timber.
The quantity of hygroscopic heat can be calculated by the known KOLLMANN formula, which expresses this quantity of heat in terms of the saturation point of the cellular membranes of the wood and of the final moisture content of the wood. This quantity of heat is generally, but not always, expressed with reference to one kilogram of anhydrous wood. For pre-seasoned timer, again of any type, the order of magnitude of the quantity of hygroscopic heat in relation to the quantity of vaporisation heat can be more than 5 percent, while for damp unseasoned wood it is less.
Another heat quantity to be considered in timber drying is the heat lost with the air which is discharged from the kiln, which is often referred to as the discharged heat. This loss of discharged heat is difficult to calculate, but is nevertheless quite considerable; for example, it is known from practical experience that it can be between 10 percent and 20 percent of the quantity of vaporisation heat.
Moreover, there is another source of heat loss in vacuum drying kilns, that is the depression heat, which is lost due to thermodynamic effects. In fact, the vaporisation heat at normal atmospheric pressure (760 torr) is 539.9 calories per liter of water evaporated, whilst at the lower pressure of 40 torr, it is 577.5 calories per liter. It can be seen that at the lower pressure there is an extra heat requirement of 37.6 calories per liter, equal to about 7%. In vacuum drying kilns there is yet another source of heat loss, that is the thermal equivalent of the work done by the vacuum pump.